In this part of the Perl Tutorial, we are going to take a look at the data structures available in Perl and how we can use them.

In Perl 5 there are basically 3 data structures. Scalars, arrays and hashes. The latter is also known as dictionaries, look-up tables or associative arrays in other languages.

Variables in Perl are always preceded with a sign called a sigil. These signs are $ for scalars, @ for arrays, and % for hashes.

A scalar can contain a single value such as a number or a string. It can also contain a reference to another data structure that we'll address later.

The name of the scalar always starts with a $ (dollar sign) followed by letters, numbers and underscores. A variable name can be $name or $long_and_descriptive_name. It can also be $LongAndDescriptiveName which is often referred-to as the "CamelCase", but the Perl community usually prefers the all-lower case variables with underscores separating the words in the name.

As we are always using strict, we always have to first declare our variables using my. (Later you will also learn about our and some other ways, but for now let's stick to the my declaration.) We can either assign a value immediately like in this example:

use strict;
use warnings;
use 5.010;

my $name = "Foo";
say $name;

or we can declare the variable first and assign only later:

use strict;
use warnings;
use 5.010;

my $name;

$name = "Foo";
say $name;

We prefer the former if the logic of the code allows it.

If we declared a variable, but have not assigned a value yet then it has a value called undef which is similar to NULL in databases, but which has slightly different behavior.

We can check if a variable is undef or not using the defined function:

use strict;
use warnings;
use 5.010;

my $name;

if (defined $name) {
  say 'defined';
} else {
  say 'NOT defined';
}

$name = "Foo";

if (defined $name) {
  say 'defined';
} else {
  say 'NOT defined';
}

say $name;

We can set a scalar variable to be undef by assigning undef to it:

$name = undef;

The scalar variables can hold either numbers or strings. So I can write:

use strict;
use warnings;
use 5.010;

my $x = "hi";
say $x;

$x = 42;
say $x;

and it will just work.

How does that work together with operators and operator overloading in Perl?

In general Perl works in the opposite way than other languages. Instead of the operands telling the operator how to behave, the operator tells the operands how they should behave.

So if I have two variables that have numbers in them then the operator decides if they really behave like numbers or if they behave like strings:

use strict;
use warnings;
use 5.010;

my $z = 2;
say $z;             # 2
my $y = 4;
say $y;             # 4

say $z + $y;        # 6
say $z . $y;        # 24
say $z x $y;        # 2222

+, the numerical addition operator, adds two numbers, so both $y and $z act like numbers.

., concatenates two strings, so both $y and $z act like strings. (In other languages you might call this the string addition.)

x, the repetition operator, repeats the string on the left hand side as many times as the number on the right hand side, so in this case $z acts as a string, and $y acts as a number.

The results would be the same if they were both created as strings:

use strict;
use warnings;
use 5.010;

my $z = "2";
say $z;             # 2
my $y = "4";
say $y;             # 4

say $z + $y;        # 6
say $z . $y;        # 24
say $z x $y;        # 2222

Even if one of them was created as a number, and the other one as a string:

use strict;
use warnings;
use 5.010;

my $z = 7;
say $z;             # 7
my $y = "4";
say $y;             # 4

say $z + $y;        # 11
say $z . $y;        # 74
say $z x $y;        # 7777

Perl automatically converts numbers to strings and strings to numbers as required by the operator.

We call numerical and string contexts.

The above cases were easy. When converting a number to a string it is just as if you put quotes around it. When converting a string to a number there are simple cases as we saw, when all the string consists of just digits. The same would happen if there was a decimal dot in the string, such as in "3.14". The question is: What if the string contained characters that are not part of any number? e.g "3.14 is pi". How would that behave in a numerical operation (aka. numerical context)?

Even that is simple, but it might need some explanation.

use strict;
use warnings;
use 5.010;

my $z = 2;
say $z;             # 2
my $y = "3.14 is pi";
say $y;             # 3.14 is pi

say $z + $y;        # 5.14
say $z . $y;        # 23.14 is pi
say $z x $y;        # 222

When a string is in numerical context Perl looks at the left side of the string, and tries to convert it to number. As long as it makes sense, that part becomes the numerical value of the variable. In numerical context (+) the string "3.14 is pi" is regarded as the number 3.14.

In a way it is completely arbitrary, but that's how it behaves so we live with that.

The above code will also generate a warning on the standard error channel (STDERR):

Argument "3.14 is pi" isn't numeric in addition(+) at example.pl line 10.

assuming you used use warnings which is highly recommended. Using it will help you notice when something is not exactly as expected. Hopefully the result of $x + $y is now clear.

Background

To be sure, Perl did not convert $y to 3.14. It just used the numerical value for the addition. This probably explains the result of $z . $y as well. In that case Perl is using the original string value.

You might wonder why $z x $y shows 222 while we had 3.14 on the right hand side, but apparently perl can only repeat a string along whole numbers... In the operation perl silently rounds the number on the right hand side. (If you are really into deep thinking, you can recognize that the "number" context mentioned earlier has actually several sub-contexts, one of them is "integer" context. In most cases perl does what would seem "the right thing" for most people who are not programmers.)

Not only that, but we don't even see the warning of the "partial string to number" conversion that we saw in the case of +.

This is not because of the difference in the operator. If we comment out the addition we will see the warning on this operation. The reason for the lack of a second warning is that when perl generated the numerical value of the string "3.14 is pi" it also stored it in a hidden pocket of the $y variable. So effectively $y now holds both a string value and a number value, and will use the right one in any new operation avoiding the conversion.

There are three more things I'd like to address. One is the behavior of a variable with undef in it, the other one is fatal warnings and the third one is avoiding the automatic "string to number conversion".

undef

If in a variable I have undef which most people would refer to as "nothing", it can still be used. In numerical context it will act as 0 in string context it will act as the empty string:

use strict;
use warnings;
use 5.010;

my $z = 3;
say $z;        # 3
my $y;

say $z + $y;   # 3
say $z . $y;   # 3

if (defined $y) {
  say "defined";
} else {
  say "NOT";          # NOT
}

With two warnings:

Use of uninitialized value $y in addition (+) at example.pl line 9.

Use of uninitialized value $y in concatenation (.) or string at example.pl line 10.

As you can see the variable is still undef at the end and thus the conditional will print "NOT".

Fatal warnings

The other thing is that some people prefer that the application will throw a hard exception instead of the soft warning. If that's your thing, you could change the beginning of the script and write

use warnings FATAL => "all";

Having that in the code, the script will print the number 3, and then throw an exception:

Use of uninitialized value $y in addition (+) at example.pl line 9.

This is the same message as was the first warning, but this time the script stops running. (Unless, of course the exception is caught, but we'll talk about that another time.)

Avoiding the automatic string to number conversion

If you would like to avoid the automatic conversion of strings when there is no exact conversion, you could check if the string looks like a number when you receive it from the outside world.

For this we are going to load the module Scalar::Util, and use the subroutine looks_like_number it supplies.

use strict;
use warnings FATAL => "all";
use 5.010;

use Scalar::Util qw(looks_like_number);

my $z = 3;
say $z;
my $y = "3.14";

if (looks_like_number($z) and looks_like_number($y)) {
  say $z + $y;
}

say $z . $y;

if (defined $y) {
  say "defined";
} else {
  say "NOT";
}

operator overloading

Finally, you could actually have operator overloading in which case the operands would tell what the operators should do, but let's leave that as an advanced topic.